Space vehicles, such as re-useable launch vehicles (RLV), often carry cryogenic fluids, such as liquid hydrogen (LH.sub.2) and liquid oxygen (LO.sub.2), into outer space for use during a mission, either as a propellant or for power generation. The storage and management of cryogenic liquids in space poses two primary problems to fluid-system designers. Firstly, due to the low-temperatures of liquid hydrogen (LH.sub.2) and liquid oxygen (LO.sub.2) heat is continuously transferred through the walls of the storage vessel into the fluid, such as during the space vehicle's orbital coast. This heat transfer can cause the liquid cryogen to boil, thus creating a gas phase and increasing the pressure inside the storage vessel. Accordingly, storage vessels are vented when the pressure reaches a predetermined level in order to maintain the structural integrity of the vessel.
The second problem involves the acquisition of a single-phase fluid from the storage vessel upon demand for use by the space vehicle. In an RLV, liquid propellants are required in orbit by the orbital maneuvering system (OMS) engines, and by the gaseous propellant supply system (GPSS) for circularization of the RLV's orbit, RLV orbital maneuvering, DC-power generation and hydraulic operations. It is therefore important to have the ability to withdraw only the liquid phase from the storage vessel on demand until the fluid has been entirely depleted. On Earth, where gravity is significant, the liquid is generally in a known location within the vessel, namely, settled against the vessel's bottom with the gas phase thereabove. In a reduced-gravity environment, however, the absence of a significant gravitational force means that the liquid and gas phases are generally free to move about inside the vessel. In other words, the liquid phase could be "floating" about the vessel distant from the liquid acquisition valves. Accordingly, fluid movement under reduced-gravity conditions hinders acquisition and withdrawal of a stored cryogenic liquid for use by the vehicle.
Such movement can also interfere with the vessel's vent operation. The vent system is most efficient if only the gas phase is vented from the vessel. Expulsion of liquid propellants through a conventional vessel vent system unnecessarily wastes the cryogenic liquid without significantly reducing the pressure inside the storage vessel.
An RLV low-gravity cryogenic propellant storage system is thus expected to supply liquid propellants to OMS engines during orbital burns and supply liquid propellants to the GPSS throughout the mission. The propellant system should also minimize propellant boil-off and venting of the storage vessel so as not to waste propellant and prevent venting of liquid-phase propellants. It is also desirable to minimize and damp sloshing of the propellant during flight.
In seeking better vessels for storing cryogenic fluids in low gravity, several storage tanks have been suggested. One such example of a cryogenic storage tank is disclosed in U.S. Pat. No. 4,412,851 to Laine which discloses a cryostat for cooling instrumentation and the like on a space craft. The cryostat is a closed storage vessel having a tubular phase separator, centrally located within the vessel, for exchanging heat with the cryogenic fluid stored therein. The phase separator consists of single tubular member having an inlet end and an outlet end. In between the inlet end and the outlet end, there is a system of two constriction sections and two obturators for withdrawing cryogenic fluid from inside the vessel in a controlled fashion. A first constriction section cools the cryogenic fluid and then isolates the fluid in a transfer chamber delimited by the obturators to allow for heat exchange between the cryogenic fluid inside the vessel and the fluid inside the transfer chamber. The obturators operate in a cyclic fashion to fill the transfer chamber, and then subsequently, expel gas from inside the chamber through the outlet end of the phase separator. The gas phase and liquid phase of the cryogenic fluid are allowed to circulate freely within the vessel relative to the transfer chamber.
Another example of a cryogenic storage tank is disclosed in U.S. Pat. No. 5,398,515 to Lak which discloses a cryogenic storage vessel having an active circulating heat transfer system for thermally destratifying both the liquid and gaseous cryogenic fluid stored in the vessel through forced convection mixing. The heat transfer system consists of a recirculation pump for circulating cryogenic fluid from inside the vessel through a spray injection system. Additionally, part of the flow from the recirculation pump may be directed through a flow control external expansion orifice for reducing the temperature of the fluid. The fluid is then directed to an internal heat exchanger of concentric tube design having an inner tube containing the spray injection flow and the outer tube containing either a parallel flow or spiral-type heat exchanger. Use of a recirculation pump in combination with a plurality of valves is expensive and creates an active system whereby fluid flow through the heat exchanger is dependent upon the reliability of the pump and valve system. It is desirable to create a passive heat transfer system for cryogenic vessels which is not reliant on an external pump.
Thus, there is a need for an improved cryogenic storage vessel for reduced-gravity environments. Such a storage vessel must be capable of effectively cooling the cryogenic fluid inside the vessel with improved heat transfer and minimal waste and also be capable of acquiring a substantially liquid phase fluid on demand for use by external devices. In addition, such a storage vessel should not be reliant on the use of an external pump and would preferably incorporate a passive heat transfer system.